"The concentrations of Na+, Ca2+, and H+ in the cytoplasm of the normal mammalian heart (Table 1) are below their electrochemical equilibrium values. Thus, energy must
be consumed to extrude these ions from the cell during the
normal contractile cycle. Intracellular [K+] is higher than
its electrochemical equilibrium value. It leaks from the cell
down its electrochemical gradient and energy is required to
pump it back to the cytoplasm. The processes that maintain
normal chemical gradients of these cations across the cell
membrane form a complex and interdependent network of
pumps, channels, and exchangers (Fig. 1)."